Automotive engine blocks are typically produced from cast iron or aluminum materials. Cast iron engine blocks are very durable and wear resistant but have the disadvantage of excessive weight. Aluminum engine blocks have the advantage of being light-weight but have the disadvantage of having poor wear and scuff resistance between the piston and rings and the mating cylinder wall. To improve wear and scuff resistance, several techniques have been used in aluminum engine blocks. The installation of cast iron cylinder liners is one technique; however, extensive machining is required to both the engine block and cylinder liner so that they fit together properly. It is also known to cast the aluminum block around a cast iron liner but this adds complexity to the casting process. Additionally, cast iron liners have the disadvantage of adding weight to the aluminum engine block. Another technique is to cast the entire aluminum block out of a high-silicon aluminum alloy. This material has excellent wear resistance but is difficult to machine and difficult to cast.
Still another technique is to cast the aluminum block out of a lower-silicon content aluminum alloy and apply a plating to the bore of the block or aluminum alloy liner to improve wear resistance. The plating is typically a nickel alloy with a controlled fine dispersion of silicon carbide or boron nitride particles distributed uniformly in the metal matrix. Plating has the disadvantage of having long cycle times and high material costs.
A further technique is to provide a thermal sprayed coating on the bore of an aluminum block that offers wear and scuff resistant properties of a cast iron liner. Thermal spraying of coatings directly on the bore has the following disadvantages:
1. Requires surface preparation of the bore prior to thermal spraying to provide a roughened surface for adhesion or bonding of the sprayed coating.
2. Periodic bond testing of coatings (which is required to insure adhesion) are typically destructive in nature and would require scrapping of the engine block.
3. Extensive masking of the engine block is required to ensure that over-spray does not come in contact with other machined surfaces.
4. Periodic checks of coating microstructure and thickness are typically destructive in nature and like the bond testing, would require scrapping of the engine block.
5. Requires preheating of the cylinder wall surfaces by flowing hot water through the engine coolant passages prior to thermal spraying, then cooling the casting during the metal spray application so as to prevent thermal damage to the casting.
6. Requires that the engine block casting be supported in a special fixture that seals the cooling passage openings to permit the flow of water through the casting.
Among the objectives of the present invention are to provide a method of making engine blocks with liners which overcomes the disadvantages of present methods; to provide an improved engine block; and to provide an improved liner.
It is a further object of this invention to provide a spray-formed liner that is light-weight when compared to cast iron liners typically used in cast aluminum blocks.
It is a further object of the present invention to provide a spray-formed cylinder bore liner for cast aluminum engines. The spray-formed liner provides wear and scuff resistance between the piston, piston rings and cylinder wall.
It is a further object of this invention to provide a spray-formed liner that requires no additional processing of the outer diameter after the thermal spray-forming of the liner. The process of thermal spray-forming a liner comprises spraying the internal diameter of a tube machined to a predetermined diameter. This results in a smooth outside diameter ready for assembly. The smooth outside diameter provides excellent heat transfer to the aluminum bore of the engine block.
Still another object of this invention is to provide a spray-formed liner that has unlimited material possibilities. The spray-formed liners are produced by a thermal spray process. Any material that can be produced in a powder or wire form for use in a thermal spray process has the potential to be used in a spray-formed liner. Material examples are metallic alloys, pure metals, clad composites, and cermets.
Yet another object of this invention is to provide a spray-formed liner that has a dual layer combination of materials. For example, an outer layer of a given material could be used on the spray-formed liner that provides excellent heat transfer while an inner layer of a given material could be used to provide wear and scuff resistance.
Still a further object of this invention is to provide a spray-formed liner that has a bonding agent or adhesive applied to the outer diameter.
In one method of assembly for the spray-formed liner the aluminum block is preheated to expand the bore of the engine block for insertion of the spray-formed liner. The block is then cooled, creating a shrink fit or compression fit around the spray-formed liner, locking it in place. Differences in coefficient of thermal expansion between the liner and aluminum bore could result in a reduced compression fit during hot engine running. In such a situation, the addition of an adhesive or bonding agent may be required to enhance the locking of the liner to the bore of the aluminum block.